Lithium Superionic Conductors Research Articles

Synthesis and electrochemical characterization of sulfur-doped Li3PO4 for all-solid-state battery applications

Isovalent and aliovalent cation-doping have been widely investigated to enhance ion transport in γ‑Li3PO4-type lithium superionic conductors (LISICONs). Anion doping, however, has been restricted to a full replacement of oxide ions by sulfide ions in some compounds (the so-called thio-LISICONs). The replacement of oxide ions by sulfide ions enhances both ion conductivity and deformability of the materials. Similar to other sulfide-type solid-electrolytes, thio-LISICONs, however, showed limited electrochemical stability against oxidation. In this report, a sonication-assisted liquid-phase synthesis approach was employed to achieve substantial sulfur‑oxygen mixing in γ-type Li3PO4. Sulfur-doped Li3PO4 possessed an ion conductivity four orders of magnitude higher than that of γ-Li3PO4, and a notably low activation energy of ion transport of 0.40(8) eV. Sulfur-oxygen mixing in sulfur-doped Li3PO4 enhanced electrochemical stability against oxidation (up to 4 V vs. Li+/Li) compared with sulfur-based Li3PS4. This study paves the way for developing mixed-anion phases in the γ-type Li3PO4 structure with enhanced electrochemical properties for all-solid-state battery applications.

Enhanced lithium separation with Li1.3Al0.3Ti1.7(PO4)3 lithium superionic conductor and aided charge balance

Traditional technologies of lithium extraction from salt lakes suffer the low efficiency of Li+/Mg2+ separation and high energy consumption. We introduce a novel electrodialysis process that leverages Li1.3Al0.3Ti1.7(PO4)3 (LATP, one of lithium superionic conductors) and incorporates an aided charge balance (ACB) system, all under low voltage and ultra-low current conditions. The remarkable ion selectivity of LATP, coupled with the enhanced recovery ratio facilitated by ACB, collectively empower this technology to achieve extremely high separation efficiency and remarkably low energy consumption. In a simulated pristine brine, the average Li/Mg separation coefficient reached 5924, accompanied by an exceptionally low energy consumption of only 0.80 kWh·kg−1Li. Furthermore, this technique enabled the production of battery-grade Li2CO3 with an outstanding purity of 99.93 %, achieved through a streamlined process comprising only two stages.

Understanding Ion Dynamics in Closoborate-Type Lithium-Ion Conductors on Different Time-Scales.

The lithium-ion transport mechanism in 0.7Li(CB9H10)-0.3Li(CB11H12) complex hydride solid electrolyte was studied over a wide time-scale (ns-ms) by choosing appropriate techniques for assessing ionic motion on the desired time-scale using nuclear magnetic resonance (NMR) relaxation, AC impedance, and pulsed field gradient-NMR (PFG-NMR) measurements. The 7Li NMR line width decreased with increasing temperature, and the spin-lattice relaxation time T1 for the cation and anions showed a minimum near 303 K, indicating that the lithium ions and the anions were highly mobile. The activation energy estimated from the analysis of the NMR relaxation time matched well with the values estimated from the AC impedance and PFG-NMR. This confirms that the lithium-ion motion in 0.7Li(CB9H10)-0.3Li(CB11H12) is the same over a wide time-scale, suggesting steady Li-ion motion over a wide transport range. This understanding offers insights into strategies for designing complex hydride lithium superionic conductors.

XRDMatch: a semi-supervised learning framework to efficiently discover room temperature lithium superionic conductors

We propose XRDMatch, a semi-supervised learning framework that integrates consistency regularization and pseudo-labeling. Using X-ray diffraction patterns as descriptors, it effectively addresses data scarcity by leveraging abundant unlabeled data.

(Battery Division Student Research Award Sponsored by Mercedes-Benz Research & Development) Rationalizing Fast Lithium-ion Diffusion in Inorganic Lithium Superionic Conductors

All-solid-state batteries are gaining attention due to the potential advantage that solid electrolytes provide in terms of safety and energy density. Understanding the mechanism of fast lithium-ion diffusion in inorganic materials has become one of the key challenges in materials science. Among various factors that affect lithium mobility, the topology of the crystal structure strongly dictates which materials can accommodate fast lithium-ion motion. Despite this, the intrinsic mechanism that connects the lithium-ion diffusion to the structural feature of the crystal structure and motion of the non-diffusing framework remains unclear, hindering the rational design of novel fast-conducting solid electrolytes.This talk focuses on the fundamental understanding of the structure-property relationship of lithium-ionic conductivities. We first discuss the structural factors governing fast lithium-ion diffusion in oxide materials. We find that both the topology of the lithium-ion diffusion network as well as the connectivity of the non-diffusing framework strongly affect lithium-ions diffusion in oxide materials. Structural features that allow lithium superionic conductivity in oxide materials are identified, which led to the discovery of 16 novel fast lithium-ion conducting frameworks.In the second part of the talk, we present our statistical framework for understanding the correlation between anion-group rotational motion and lithium-ion translational motion. Using event-detection algorithms on long ab-initio molecular dynamics trajectories, we detect and differentiate various types of rotational motion of anion groups. This allows us to obtain a statistically rigorous understanding of how each type of anion group rotational motion affects lithium-ion diffusion events. These fundamental understandings provide design guidelines towards the development of fast-diffusing inorganic materials optimal for all-solid-state batteries.

Design principles for sodium superionic conductors

Motivated by the high-performance solid-state lithium batteries enabled by lithium superionic conductors, sodium superionic conductor materials have great potential to empower sodium batteries with high energy, low cost, and sustainability. A critical challenge lies in designing and discovering sodium superionic conductors with high ionic conductivities to enable the development of solid-state sodium batteries. Here, by studying the structures and diffusion mechanisms of Li-ion versus Na-ion conducting solids, we reveal the structural feature of face-sharing high-coordination sites for fast sodium-ion conductors. By applying this feature as a design principle, we discover a number of Na-ion conductors in oxides, sulfides, and halides. Notably, we discover a chloride-based family of Na-ion conductors NaxMyCl6 (M = La–Sm) with UCl3-type structure and experimentally validate with the highest reported ionic conductivity. Our findings not only pave the way for the future development of sodium-ion conductors for sodium batteries, but also consolidate design principles of fast ion-conducting materials for a variety of energy applications.

Machine learning molecular dynamics simulation identifying weakly negative effect of polyanion rotation on Li-ion migration

Understanding the physical picture of Li ion transport in the current ionic conductors is quite essential to further develop lithium superionic conductors for solid-state batteries. The traditional practice of directly extrapolating room temperature ion diffusion properties from the high-temperature (>600 K) ab initio molecular dynamics simulations (AIMD) simulations by the Arrhenius assumption unavoidably cause some deviations. Fortunately, the ultralong-time molecular dynamics simulation based on the machine-learning interatomic potentials (MLMD) is a more suitable tool to probe into ion diffusion events at low temperatures and simultaneously keeps the accuracy at the density functional theory level. Herein, by the low-temperature MLMD simulations, the non-linear Arrhenius behavior of Li ion was found for Li3ErCl6, which is the main reason for the traditional AIMD simulation overestimating its ionic conductivity. The 1μs MLMD simulations capture polyanion rotation events in Li7P3S11 at room temperature, in which four [PS4]3− tetrahedra belonging to a part of the longer-chain [P2S7]4− group are noticed with remarkable rotational motions, while the isolated group [PS4]3− does not rotate. However, no polyanion rotation is observed in Li10GeP2S12, β-Li3PS4, Li3ErCl6, and Li3YBr6 at 300 K during 1μs simulation time. Additionally, the ultralong-time MLMD simulations demonstrate that not only there is no paddle-wheel effect in the crystalline Li7P3S11 at room temperature, but also the rotational [PS4]3− polyanion groups have weakly negative impacts on the overall Li ion diffusion. The ultralong-time MLMD simulations deepen our understanding of the relationship between the polyanion rotation and cation diffusion in ionic conductors at room environments.

Preparation of Metal-Oxide-Doped Li7P2S8Br0.25I0.75 Solid Electrolytes for All-Solid-State Lithium Batteries.

The all-solid-state lithium battery (ASSB) has received great attention due to its greater safety than the conventional lithium-ion battery (LIB). Sulfide-based inorganic solid electrolytes are an important component to fabricate the ASSB. But to attain a better performance, the ionic conductivity and electrochemical stability of the sulfide-based solid electrolytes need to be improved. In this work, we prepared the metal-oxide-doped/mixed Li7P2S8I0.75Br0.25 lithium superionic conductors by a dry ball-milling process. The high ionic conductivity was achieved by a low-temperature (200 °C) heat-treatment process. The metal-oxide-doped Li7P2S8I0.75Br0.25 solid electrolyte exhibited a higher ionic conductivity value of 7.3 mS cm-1 at room temperature than the bare Li7P2S8I0.75Br0.25 solid electrolyte. The structural characteristics of the prepared solid electrolytes were studied by solid NMR and laser Raman analysis. The electrochemical stability of the prepared solid electrolyte was studied by cyclic voltammetry and DC charge-discharge analysis. The addition of metal oxide increased the electrochemical stability and dry-air stability of the Li7P2S8I0.75Br0.25 solid electrolyte. The Ta2O5-doped Li7P2S8I0.75Br0.25 solid electrolyte was stable even after 300 charge-discharge DC cycles and also 100 h of dry-air exposure. Further, the Ta2O5-doped Li7P2S8I0.75Br0.25 solid electrolyte-based ASSB exhibited a high discharge capacity value of 184 mA h g-1 at 0.1 C rate with 66% initial cycle Coulombic efficiency.

Frustration in Super-Ionic Conductors Unraveled by the Density of Atomistic States.

The frustration in super-ionic conductors enables their exceptionally high ionic conductivities, which are desired for many technological applications including batteries and fuel cells. A key challenge in the study of frustration is the difficulties in analyzing a large number of disordered atomistic configurations. Using lithium super-ionic conductors as model systems, we propose and demonstrate the density of atomistic states (DOAS) analytics to quantitatively characterize the onset and degree of disordering, reveal the energetics of local disorder, and elucidate how the frustration enhances diffusion through the broadening and overlapping of the energy levels of atomistic states. Furthermore, material design strategies aided by the DOAS are devised and demonstrated for new super-ionic conductors. The DOAS is generally applicable analytics for unraveling fundamental mechanisms in complex atomistic systems and guiding material design.

Improved electrochemical and air stability performance of SeS2 doped argyrodite lithium superionic conductors for all-solid-state lithium batteries

At present, sulfide-based solid electrolytes have attracted considerable attention, especially argyrodite-type solid electrolyte, Li6PS5X (X = Cl, Br, I), due to its appropriate mechanical strength and high ionic conductivity (>10–3 S/cm). However, it still falls short of liquid electrolytes in terms of ionic conductivity and has the fatal drawback of being unstable to moist air. In this report, we prepare SeS2 doped Li6PS5Cl through high-energy ball milling followed by heat treatment. We also confirm the structural properties using powder X-ray diffraction (XRD) and Raman analyses. Next, we perform surface morphology and elemental analysis using a field emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscope (EDS). The synthesized Li6.03P0.97Se0.03S5Cl electrolyte shows a higher ionic conductivity of 5.4 mS/cm and a lower activation energy of 0.287 eV compared to pristine Li6PS5Cl (4.4 mS/cm, 0.292 eV) at 25 ℃, and it also shows good stability against Li metal. The optimized electrolyte shows a higher initial discharge capacity of 175 mAh/g and coulombic efficiency of 67.6%. Interestingly, Li6.03P0.97Se0.03S5Cl showed 61.5% of the 1 C rate capacity compared to that at the 0.05 C rate, which was significantly improved compared to the corresponding value of the pristine. Finally, to measure the air stability properties, the H2S generation amount test and EIS test were both performed before and after exposure to air and Li6.03P0.97Se0.03S5Cl shows a high ionic conductivity of 3.06 mS/cm after 30 min dry air exposure.

Anharmonic Cation-Anion Coupling Dynamics Assisted Lithium-Ion Diffusion in Sulfide Solid Electrolytes.

Sulfide-based lithium superionic conductors often show higher Li-ion conductivity than other types of electrolyte materials. This work unveils a unique Li-ion conductive behavior in these materials through the perspective of anharmonic coupling assisted Li-ion diffusion. Li hopping events can happen simultaneously with various types of lattice dynamics, while only a statistically important synchronization of motions may indicate coupling. This method enables a direct evaluation of the coupling strength between these motions, which more fundamentally decides if a specific type of lattice motion is really anharmonically coupled to the Li hopping event and whether the coupling can facilitate the Li diffusion. By a new ab initio computational approach, this work unveils a unique phenomenon in prototype sulfide electrolytes in comparison with typical halide ones, that Li-ion conduction can be boosted by the anharmonic coupling of low-frequency Li phonon modes with high-frequency anion stretching or flexing phonon modes, rather than the low-frequency rotational modes. The coupling pushes Li ions toward the diffusion channels for reduced diffusion barriers. The result from the lower temperature range (≈0-300 K) of simulation can also be more relevant to the application of solid-state batteries.

Lithium superionic conductors with corner-sharing frameworks.

Superionic lithium conductivity has only been discovered in a few classes of materials, mostly found in thiophosphates and rarely in oxides. Herein, we reveal that corner-sharing connectivity of the oxide crystal structure framework promotes superionic conductivity, which we rationalize from the distorted lithium environment and reduced interaction between lithium and non-lithium cations. By performing a high-throughput search for materials with this feature, we discover ten new oxide frameworks predicted to exhibit superionic conductivity-from which we experimentally demonstrate LiGa(SeO3)2 with a bulk ionic conductivity of 0.11 mS cm-1 and an activation energy of 0.17 eV. Our findings provide insight into the factors that govern fast lithium mobility in oxide materials and will accelerate the development of new oxide electrolytes for all-solid-state batteries.

High-Entropy Polyanionic Lithium Superionic Conductors

High-entropy ceramics are attracting large interest because of their unique materials properties. Nevertheless, the effect of entropy on the lithium transport remains largely elusive. Here, we report, for the first time, about medium- and high-entropy polyanionic lithium superionic conductors crystallizing in the F–43m space group and adopting the so-called argyrodite structure. The Li6PS5[Cl0.33Br0.33I0.33], Li6P[S2.5Se2.5][Cl0.33Br0.33I0.33], and Li6.5[Ge0.5P0.5][S2.5Se2.5][Cl0.33Br0.33I0.33] materials were structurally characterized using complementary synchrotron and neutron scattering techniques in combination with 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. We show that, in contrast to other high-entropy ceramics, an unequal distribution of elements over the respective crystallographic sites occurs in these materials. Using electrochemical impedance spectroscopy (EIS) and 7Li pulsed field gradient (PFG) NMR spectroscopy, we demonstrate that introducing entropy (compositional disorder) marginally affects the room-temperature ionic conductivity (∼10–3 S cm–1) but instead lowers the activation energy for conduction to 0.22 eV. Our results emphasize the possibility of increasing entropy in polyanionic materials, thereby opening up compositional space for the search of Li-ion conductors with unprecedented properties.

Role of Critical Oxygen Concentration in the β-Li3PS4–xOx Solid Electrolyte

Lithium superionic conductors are the critical enabling component for next-generation all-solid lithium-ion batteries. In particular, the β polymorph of Li3PS4 has attracted major interest due to its combination of excellent ionic conductivity and passivating interfacial stability with Li. In this work, we systematically investigated the effect of oxygenation in β-Li3PS4 to further enhance its ionic conductivity and electrochemical stability using density functional theory calculations and ab initio molecular dynamics simulations. We predict that a maximum ionic conductivity of 1.52 mS cm–1 (and minimum activation energy) can be achieved at x = 0.25 in Li3PS4–xOx which is about 7 times higher than that of β-Li3PS4. This increase in ionic conductivity can be attributed to the flattening of the potential energy surface due to the diversification of the Li chemical environments by the S–O mixed-anionic framework, resulting in a change from quasi-2D to 3D Li diffusion. We highlight that the spatial localization of the electrostatic potential is a qualitative descriptor to assess the migration barrier of the charge carrier in the S–O mixed framework. These microscopic analyses shed light on the role of critical oxygen concentration to tune the rate-performance of mixed-anion lithium superionic conductors.

Improved ionic conductivity and structural transition from (nLi2S-LiI)-(P2S5) solid solutions to LixP2SyI crystalline electrolytes

In this study, LixP2SyI – type crystalline lithium superionic conductors have been synthesized from nLi2S-LiI (n = 1, 2, 3, and 4) solid solutions and P2S5 using the high energy ball milling process. First, different concentrations of Li2S were mixed and ball-milled with LiI at different molar ratios to obtain various Li2S-LiI solid solutions (nLi2S-LiI). Then, P2S5 was added to the prepared nLi2S-LiI solid solutions to get crystalline solid electrolytes. The crystalline nature and the composition of the prepared nLi2S-LiI solid solutions, and P2S5 added nLi2S-LiI (LixP2SyI) crystalline solids were examined using the powder X-ray diffraction technique. The evolution of the crystalline structural units was captured by laser Raman spectroscopy analysis. The surface morphology changes with varying Li2S concentrations as well as the addition of P2S5 were studied using field emission scanning electron microscopy. Electrochemical impedance spectroscopy analysis revealed that the prepared Li5S2I (3.1 ×10−5 S cm−1) and Li7P2S8I (1.07 ×10−3 S cm−1) solid electrolytes exhibited higher ionic conductivities than the other nLi2S-LiI solid solutions and LixP2SyI crystalline solids, respectively. The result of cyclic voltammetry analysis and DC polarization analysis proved that the prepared nLi2S-LiI solid solutions and LixP2SyI crystalline solids are stable against lithium metal. All-solid-state lithium batteries fabricated using nLi2S-LiI solid solutions and LixP2SyI crystalline solid electrolytes showed comparable electrochemical performances. In the present study, we demonstrated nLi2S-LiI solid solutions and LixP2SyI crystalline solids which could potentially be used as solid electrolytes for the fabrication of ASSBs with good performance.

Lithium Oxide Superionic Conductors Inspired by Garnet and NASICON Structures

AbstractThe key component in lithium solid‐state batteries (SSBs) is the solid electrolyte composed of lithium superionic conductors (SICs). Lithium oxide SICs offer improved electrochemical and chemical stability compared with sulfides, and their recent advancements have largely been achieved using materials in the garnet‐ and NASICON (sodium superionic conductor)‐ structured families. In this work, using the ion‐conduction mechanisms in garnet and NASICON as inspiration, a common pattern of an “activated diffusion network” and three structural features that are beneficial for superionic conduction: a 3D percolation Li diffusion network, short distances between occupied Li sites, and the “homogeneity” of the transport path are identified. A high‐throughput computational screening is performed to search for new lithium oxide SICs that share these features. From this search, seven candidates are proposed exhibiting high room‐temperature ionic conductivity evaluated using ab initio molecular dynamics simulations. Their structural frameworks including spinel, oxy‐argyrodite, sodalite, and LiM(SeO3)2 present new opportunities for enriching the structural families of lithium oxide SICs.

Preparation of highly conductive metal doped/substituted Li7P2S8Br(1-x)Ix type lithium superionic conductor for all-solid-state lithium battery applications

Sulfide-based lithium superionic conducting solid electrolytes for all-solid-state lithium batteries have received much attention due to their high safety, ionic conductivity, and compatibility. However, high-temperature treatment over a long time is needed to achieve high ionic conductivity. In this work, we have prepared the dual-halide-based Li7P2S8X type lithium superionic conductors by dry ball mill process and the high ionic conducting phase was achieved by low temperature (200℃) heat-treatment process. High ionic conducting halogen-halogen composition was arrived at through a series of optimization processes. Among the compositions tested, the Li7P2S8Br0.25I0.75 solid electrolyte exhibited a high ionic conductivity value of 6.16 mS cm−1, at room temperature. The ionic radius of the halogen element played an important role in the formation of the high ionic conducting phase. Further, the ionic conductivity of Li7P2S8I0.75Br0.25 solid electrolyte was improved by metal doping at P-site. The Sn-doped Li7P2S8I0.75Br0.25 solid electrolyte exhibited a high ionic conductivity value of 7.78 mS cm−1, at room temperature. The addition of large-sized Sn atoms extended the crystal lattice parameters and increased the Li-ions transport path. The partial substitution of Sn atoms at P-site was confirmed by NMR and Laser Raman analysis. The electrochemical stability of the prepared solid electrolyte was studied by cyclic voltammetry and DC charge-discharge analysis. The addition of a metal atom to the Li7P2S8Br0.25I0.75 solid electrolyte decreased the side reaction with the Li metals. Thus, low-temperature treated metal-doped Li7P2S8Br0.25I0.75 solid electrolytes are highly favourable for the fabrication of high-performance all-solid-state batteries. The fabricated all-solid-state battery using Sn doped Li7P2S8Br0.25I0.75 solid electrolyte exhibited the high specific capacity value of 170 mAh g−1 (0.1C), at room temperature.

Investigation of Polymer/Ceramic Composite Solid Electrolyte System: The Case of PEO/LGPS Composite Electrolytes

The incorporation of inorganic lithium superionic conductors in polymer/ceramic composite electrolytes has been frequently proposed since this approach is expected to take advantage of the high ionic conductivities of the lithium superionic conductors and the elasticity of the polymer constituents of the composites. Nevertheless, the properties and mechanisms of polymer/ceramic composite electrolytes are yet to be comprehensively investigated. In this work, we systematically study sulfide-based polymer/ceramic composites from the aspects of composition dependence, electrochemical performance, and chemical stability. The composition-dependent Li-ion conduction mechanism and electrochemical behavior have been revealed for polyethylene oxide/Li10GeP2S12 composite electrolytes, highlighting the rational selection of compositions of polymer/ceramic composites toward desired functions. Furthermore, the chemical stability of the sulfide electrolyte in diverse solvent media as well as the potential internal reactions between the components of the composite electrolyte have been investigated, which underline the chemical stability consideration in the design and fabrication of the composite electrolyte. Thus, this work aims at contributing to the design and fabrication of sulfide-based polymer/ceramic composite electrolytes that enable high-performance lithium metal batteries.

Bridging the gap between simulated and experimental ionic conductivities in lithium superionic conductors

Lithium superionic conductors (LSCs) are of major importance as solid electrolytes for next-generation all-solid-state lithium-ion batteries. While ab initio molecular dynamics have been extensively applied to study these materials, there are often large discrepancies between predicted and experimentally measured ionic conductivities and activation energies due to the high temperatures and short time scales of such simulations. Here, we present a strategy to bridge this gap using moment tensor potentials (MTPs). We show that MTPs trained on energies and forces computed using the van der Waals optB88 functional yield much more accurate lattice parameters, which in turn leads to accurate prediction of ionic conductivities and activation energies for the Li0·33La0·56TiO3, Li3YCl6 and Li7P3S11 LSCs. NPT MD simulations using the optB88 MTPs also reveal that all three LSCs undergo a transition between two quasi-linear Arrhenius regimes at relatively low temperatures. This transition can be traced to an increase in the number and diversity of diffusion pathways, in some cases with a change in the dimensionality of diffusion. This work presents not only an approach to develop high accuracy MTPs, but also outlines the diffusion characteristics for LSCs which is otherwise inaccessible through ab initio computation.

Lithium‐Metal Anode Instability of the Superionic Halide Solid Electrolytes and the Implications for Solid‐State Batteries

AbstractOwing to high ionic conductivity and good oxidation stability, halide‐based solid electrolytes regain interest for application in solid‐state batteries. While stability at the cathode interface seems to be given, the stability against the lithium metal anode has not been explored yet. Herein, the formation of a reaction layer between Li3InCl6 (Li3YCl6) and lithium is studied by sputter deposition of lithium metal and subsequent in situ X‐ray photoelectron spectroscopy as well as by impedance spectroscopy. The interface is thermodynamically unstable and results in a continuously growing interphase resistance. Additionally, the interface between Li3InCl6 and Li6PS5Cl is characterized by impedance spectroscopy to discern whether a combined use as cathode electrolyte and separator electrolyte, respectively, might enable long‐term stable and low impedance operation. In fact, oxidation stable halide‐based lithium superionic conductors cannot be used against Li, but may be promising candidates as cathode electrolytes.